This relates to Fourier transform spectroscopy and in particular, to a method and apparatus for correcting non-linearities in a detector and associated electronics of a Fourier transform spectrometer.
The general principles of Fourier transform spectroscopy are well known. In the typical spectrometer, two coherent beams of electromagnetic radiation are combined after traversing different optical paths so as to produce an interference pattern. The intensity in the interfering pattern varies in a manner that depends on the spectrum of the interfering beams. By recording the intensity as a function of the path difference between the two interfering beams, the power spectrum can be deduced. See for example, G. R. Fowles, Introduction to Modern Optics (Holt, Rhinehart & Winston, 1968).
Fourier transform infrared spectrometers are commercially available. Typically, they include a source of radiation, an interferometer, a sample compartment, a detector on which an interferogram is formed and signal processing electronics for processing an electronic signal representative of the interferogram and for Fourier transforming said signal. In present day systems typical detectors are a high sensitivity thermal DTGS detector or a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector.
The resulting Fourier transform is known to be inaccurate because of non-linearities in the detector and in the signal processing circuitry associated therewith. While compensation techniques exist for reducing these inaccuracies (see, for example, U.S. Pat. No. 4,682,022), these techniques are not altogether satisfactory. They are not as accurate as might be desired and they tend to be demanding of the available signal processing resources. In particular, if the detector is ac-coupled, non-linearities may not easily be eliminated by a simple correction circuitry or algorithm since the light intensity on the detector is not known. Moreover, this correction has to be adapted to the individual detector, in general requiring an individual calibration process.